Current trends in portable and mobile communications are towards greater numbers of different systems often with wider bandwidths, operating within a single device. Antenna systems for these devices need to be capable of operating over a wide frequency range or multiple frequency bands, typically between 900MHz and 6GHz (e.g. GSM, IMT-2000, Wi-Fi and WiMax). Portable and mobile applications also require antennas to be optimised with respect to radiation pattern, efficiency and physical size. A greater understanding of the characteristics and design challenges of broadband and multi-band antennas forms the core motivation of this work. The antennas developed as part of this work are all related to the planar monopole antenna: classical planar monopole antennas and printed planar monopoles. A number of techniques to extend the impedance bandwidth of the planar monopole antenna such as bevelling were applied, producing impedance bandwidth ratios up to 10:1 in some cases (suitable for ultra-wideband applications), and multi-band responses in others. Techniques to reduce the antenna size or profile were also implemented. These included the use of short circuits between the antenna and its groundplane, and folding. These techniques achieved significant reductions in antenna height (up to 50%) or in lower edge frequency (typically 30%). A novel corrugation technique was also used to reduce antenna height by up to 30%. Improvements in the omni-directionality and stability of the radiation characteristics have been achieved by modifying the antenna geometry. Very low profile printed planar monopoles, implemented in both microstrip fed and single-sided co-planar waveguide fed formats, are also investigated. Fractional impedance bandwidths in excess of 100% as well as multi-band configurations with potential reconfigurable applications were achieved with these antennas.